This invention relates to projection lenses and, in particular folded, telecentric projection lenses for use in forming an image of an object composed of pixels, such as, a DMD, a reflective LCD, a transmissive LCD, or the like.
As used in this specification and in the claims, the following terms shall have the following meanings:
(1) Telecentric
Telecentric lenses are lenses which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinity.
In practical applications, a telecentric pupil need not actually be at infinity since a lens having an entrance or exit pupil at a sufficiently large distance from the lens"" optical surfaces will in essence operate as a telecentric system. The principal rays for such a lens will be substantially parallel to the optical axis and thus the lens will in general be functionally equivalent to a lens for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the terms xe2x80x9ctelecentricxe2x80x9d and xe2x80x9ctelecentric lensxe2x80x9d are intended to include lenses which have a pupil at a long distance from the lens"" elements, and the term xe2x80x9ctelecentric pupilxe2x80x9d is used to describe such a pupil at a long distance from the lens"" elements. For the projection lenses of the invention, the telecentric pupil distance will in general be at least about 20 times the lens"" focal length.
(2) Effective Back Focal Length
The effective back focal length (BFL) of a projection lens/pixelized panel combination is the distance between the front surface of the pixelized panel and the vertex of the back surface of the rearward-most lens element of the projection lens which has optical power when (1) the image of the pixelized panel is located at infinity and (2) the projection lens is located in air, i.e., the space between the rearward-most lens element of the projection lens and the pixelized panel is filled with air as opposed to the glasses making up the prisms, beam splitters, etc. normally used between a projection lens and a pixelized panel.
A. Projection Systems
Projection systems are used to form an image of an object on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
The basic structure of such a system is shown in FIG. 4, where 10 is a light source (e.g., a metal halide or a high pressure mercury vapor lamp), 12 is illumination optics which forms an image of the light source (the xe2x80x9coutputxe2x80x9d of the illumination system), 14 is the object which is to be projected (i.e., for the lenses of the present invention, a matrix of on and off pixels), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on viewing screen 16.
For front projection systems, the viewer will be on the left side of screen 16 in FIG. 4, while for rear projection systems, the viewer will be on the right side of the screen. For rear projection systems which are to be housed in a single cabinet, one or more mirrors are often used between the projection lens and the screen to fold the optical path and thus reduce the system""s overall size.
Projection systems in which the object is a pixelized panel (also known in the art as a xe2x80x9cdigital light valvexe2x80x9d or a xe2x80x9cmicrodisplayxe2x80x9d) are used in a variety of applications. Such systems preferably employ a single projection lens which forms an image of a single panel used to produce (either sequentially or simultaneously) the red, green, and blue components of the final image or, in some cases, an image of three panels, one for red light, a second for green light, and a third for blue light. For certain applications, e.g., large image rear projection systems, multiple panels and multiple projection lenses are used, with each panel/projection lens combination producing a portion of the overall image. Irrespective of the details of the application, the projection lens generally needs to have a long effective back focal length to accommodate the prisms, beam splitters, and other components normally used with pixelized panels.
A particularly important application of projection systems employing pixelized panels is in the area of rear projection systems which can used as large screen projection televisions (PTVs) and/or computer monitors. Improvements in the technology used to manufacture microdisplays has led to a rise in the popularity of projection systems employing such displays. To compete effectively with the established cathode ray tube (CRT) technology, projection systems based on microdisplays need to be smaller in size and lower in weight than CRT systems having the same screen size.
B. Optical Performance
To display images having a high information content (e.g., to display data), a microdisplay must have a large number of pixels. Since the devices themselves are small, the individual pixels are small, a typical pixel size ranging from 17xcexc for DMD displays to approximately 8xcexc or even less for reflective LCDs. This means that the projection lenses used in these systems must have a very high level of correction of aberrations. Of particular importance is the correction of chromatic aberrations and distortion.
A high level of chromatic aberration correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing CRTs a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction, including correction of secondary lateral color, is thus needed from the projection lens.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center.
Moreover, projection lenses are often used with offset panels. In particular, in the case of DMDs, an offset is typically needed to provide the appropriate illumination geometry and to allow the dark-field light to miss the entrance pupil of the lens. This dark field light corresponds to the off position of the pixels of the DMD.
When a panel is offset the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
Low distortion and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of distortion or color aberration in the images of such interfaces.
In addition to high levels of color and distortion correction, projection lenses for use with pixelized panels need to have low levels of ghost generation, especially when the pixelized panel is of the reflective type, e.g., a DMD or reflective LCD.
As known in the art, ghosts can be generated by image light reflecting back towards the object from one of the lens surfaces of a projection lens. Depending upon the shape of the lens surface and its location relative to the object, such reflected light can be re-reflected off of the object so that it reenters the projection lens and is projected onto the screen along with the desired image. Such ghost light always reduces contrast at least to some extent. In extreme cases, a second image can actually be seen on the screen.
Because the operation of DMDs and reflective LCDs depend upon their ability to reflect light efficiently, projection systems employing panels of these types are particularly susceptible to ghost problems. Ghosts can also be generated by light reflecting backwards off of one lens surface and then being re-reflected in a forward direction by a second lens surface. When reflective pixelized panels are used, ghosts generated by reflections from two lens surfaces are generally less troublesome than ghosts generated by a lens surface/pixelized panel combination.
C. Telecentricity
The above-mentioned pixelized panels and, in particular, DMDs, typically require that the light beam from the illumination system has a near-normal angle of incidence upon the display.
In terms of the projection lens, this translates into a requirement that the lens has a telecentric entrance pupil, i.e., the projection lens must be telecentric in the direction of its short imaging conjugate where the object (pixelized panel) is located. This makes the lens asymmetric about the aperture stop which makes the correction of lateral color more difficult.
D. Cabinet Size
For rear projection systems, there is an ever increasing demand for smaller cabinet sizes (smaller footprints).
In terms of the projection lens, this translates into a requirement that the lens has a wide field of view in the direction of the image (screen). Increases in the field of view from, for example, 82xc2x0 to, for example, 88xc2x0, can be of substantial significance to manufacturers of projection televisions. This is so because such an increase in the field of view of the projection lens can allow the TV manufacturer to reduce the dimensions of its cabinet by an inch or more. A smaller cabinet, in turn, makes a projection television more desirable in the highly competitive consumer market for PTVs.
The requirement for a large field of view makes it even more difficult to correct the lateral color of the lens. This is especially so when combined with the requirement for a long effective back focal length which itself makes it more difficult to correct lateral color. Also, as mentioned above, the requirement for telecentricity is a third factor which makes the correction of lateral color challenging.
In addition to increasing the field of view, cabinet sizes can also be i reduced through the use of a folded projection lens, i.e., a lens having an internal reflective surface (e.g. a mirror or prism) which allows the lens to have an overall form which is easier to integrate with the other components of the projection system and is more compact. In terms of lens design, the use of such a reflective surface means that two of the lens units making up the projection lens must be separated by a distance which is sufficiently long to receive the reflective surface. A construction of this type makes it more difficult to correct the aberrations of the system, especially if the lens is to include only a small number of lens elements as is desired to reduce the cost, weight, and complexity of the projection lens.
Achieving a long back focal length, a wide field of view in the direction of the lens"" long conjugate, telecentricity, and a folded configuration, while still maintaining high levels of aberration correction and low levels of ghost generation, is particularly challenging since these various requirements tend to work against one another. To do so while minimizing the number of lens elements used in the lens is even more demanding. As discussed and illustrated below, the present invention provides projection lenses which satisfy these conflicting criteria.
In view of the foregoing, there exists a need in the art for projection lenses for use with pixelized panels which have some and preferably all of the following properties:
(1) a high level of lateral color correction, including correction of secondary lateral color;
(2) low distortion;
(3) a large field of view in the direction of the image;
(4) a telecentric entrance pupil;
(5) a long effective back focal length;
(6) a folded configuration;
(7) a low level of ghost generation; and
(8) a low element count.
To satisfy this need in the art, the invention provides projection lenses which have some and preferably all of the above eight features.
In particular, the invention provides a projection lens for forming an enlarged image of a pixelized panel on a screen, said projection lens having an optical axis, a long conjugate side (image or screen side), a short conjugate side (object or pixelized panel side), and an effective focal length f0, said lens comprising the following in order from the long conjugate side to the short conjugate side:
(A) a first lens unit (U1) having a negative power and comprising a plurality of lens elements, wherein:
(i) one of the lens elements LM is a negative lens element of overall meniscus shape which is convex towards the long conjugate side and comprises at least one aspheric surface; and
(ii) another of the lens elements (e.g., lens element LS) has an optical surface S1 which constitutes the short conjugate end of the first lens unit;
(B) a reflective surface (RS) for folding the projection lens"" optical axis (e.g., a mirror or prism which produces a fold in the optical axis in the range of, for example, 60-70xc2x0, e.g., approximately 64xc2x0); and
(C) a second lens unit (U2) having a positive power and comprising a plurality of lens elements, wherein:
(i) one of the lens elements LA is a positive lens element which comprises at least one aspheric surface; and
(ii) another of the lens elements (e.g., L4) has an optical surface S2 which constitutes the long conjugate end of the second lens unit;
wherein:
(a) the first and second lens units are the only lens units of the projection lens;
(b) the projection lens has a field of view in the direction of the long conjugate which is greater than 82 degrees and preferably greater than or equal to 85 degrees (e.g., a field of view of 88 degrees);
(c) the projection lens is telecentric on the short conjugate side;
(d) the projection lens has an effective back focal length BFL which satisfies the relationship:
BFL/f0 greater than 2.0;
(e) the projection lens has a mechanical spacing S1-2 between S1 and S2 which satisfies the relationship:
S1-2/f0 greater than 3.5,
where the mechanical spacing is the smaller of the center-to-center distance and the edge-to-edge distance between S1 and S2 for an unfolded optical axis; and
(f) the projection lens has a lateral color LC in its short conjugate focal plane which satisfies the relationships:
LCred-blue less than 0.0012xc2x7f0 (preferably less than 0.001xc2x7f0),
LCred-green less than 0.0012xc2x7f0 (preferably less than 0.001xc2x7f0), and
LCblue-green less than 0.0012xc2x7f0 (preferably less than 0.001xc2x7f0),
where (i) the red-blue and red-green relationships are satisfied over the full field in the short conjugate focal plane, (ii) the blue-green relationship is satisfied over at least 95% of the full field in the short conjugate focal plane, and (iii) the red, green, and blue wavelengths are 0.62 micrometers, 0.55 micrometers, and 0.46 micrometers, respectively.
Preferably, the BFL/f0 ratio is greater than 2.5. Similarly, the S1-2/f0 ratio is preferably greater than 4.0 and most preferably greater than 4.5.
Also, in addition to providing a large space between the short conjugate end of the first lens unit and the long conjugate end of the second lens unit, the projection lenses of the invention also preferably provide a large space between the short conjugate end of first lens unit (i.e., the S1 surface) and the projection lens"" aperture stop. In particular, the spacing S1-AS between the center of optical surface S1 and the center of the aperture stop preferably satisfies the relationship:
S1-AS/f0 greater than 3.5.
It should be noted that the projection lens can have a physical aperture stop or can use the output of the illumination system as a virtual aperture stop. In either case, the aperture stop is preferably on the short conjugate side of the reflective surface. Alternatively, but less preferred, the aperture stop can be located at the reflective surface, e.g., an aperture stop can be applied to or painted onto the reflective surface. Note that for the projection lens to operate efficiently, the aperture stop should either completely clear the reflective surface or should be completely on the reflective surface, i.e., the reflective surface should not intersect and thus cut off a part of the aperture stop.
Although an aperture stop on the long conjugate side of the reflective surface can be used in the practice of the invention, such a location for the aperture stop is generally not preferred since the second lens unit then must have a long focal length to produce a telecentric entrance pupil for the overall lens.
In terms of distortion, the projection lenses of the invention preferably have a percentage distortion D which:
(i) over the full field has a magnitude that is less than 1.0 (i.e., at all points of the field the magnitude of the distortion is less than 1.0%); and
(ii) over the half field-to-full field range has a maximum value Dmax and a minimum value Dmin which satisfy the relationship:
|Dmaxxe2x88x92Dmin| less than 0.4.
The second of these criteria for a high level of distortion correction is directed to the phenomenon known as xe2x80x9capparent distortion.xe2x80x9d When looking at an image on a screen, users are particularly sensitive to curvature along the top or bottom of the image. Such curvature will arise if the distortion varies between, for example, the middle of the top of the screen to the edges of the top of the screen. For a typical 16:9 format, the middle of the top of the screen corresponds to the half field of view and the edges of the top of the screen correspond to the full field of view. By keeping the variation in percentage distortion over this range below 0.4, the problems of apparent distortion is avoided.
Preferably, the projection lenses of the invention achieve the above features of the invention which less than eight lens elements. Most preferably, two of the lens elements are in the form of a doublet so that assembly of the lens requires positioning of less than seven lens components.
Additional features and advantages of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention.